Hello Everyone,
Hope you all are fine, I’m student of MS Electrical Engg, working on my semester project. I’m using NC7SB3157 Switch input pin S (is ouput of my controller), Vcc is 5V, Pin A is output of my switch which is connected to external connector, Pin B0 is short with ground and Pin B1 is pull down using 50 M Ohm resistance. When Switch input S is High A is connected to B1, but during off state this output pin resistance is 430 K Ohm but I want Open circuit (Infinite resistance) in off state what Should I do to get infinite resistance in off state, or anyone can please suggest me any other SPDT switch with same foot print, You can suggest me BJT, CMOS switches and optocouplers also so my problem is solved. Waiting for your replies
Hi usamaajaz87262,
Welcome to the Tech Forum!
A couple of points/questions:
First, you’ll never get a solid-state switch to have infinite resistance when open - there is always some leakage, so if you need that, this is not a suitable option.
Second, the NC7SB3157 comes in more than one package type. Which one do you have?
Edit: Looking at the specs in the datasheet, they do not specify “Output pin resistance”. They do specify “Off State Leakage Current” which would be related to off state resistance, though it would be dependent on the values of A and B at which it is tested. So how did you determine the 430K Ohm value of “output pin resistance”? What was your test method?
Hello @David_1528,
What you suggest instead of switch to have infinite resistance in Power Off state,
I’m using 6-Lead, MicroPak 1.0mm Wide Package
They didn’t specify output pin resistance, off state leakage current is 1-2uA and I think this resistance coming from leakage current
Hi usamaajaz87262,
The only way to get essentially infinite off-state resistance (well, at least a few orders of magnitude higher resistance) is with a mechanical switch, like a small mechanical relay or reed relay, which of course will be physically much larger. With that said, I calculate a higher impedance between input to output than the 430K Ohms you state. That’s why I asked how you determined that value.
The datasheet states that off-state leakage current, with A as low as 0V and B as high as Vcc, is a maximum of 0.1uA at 25°C, or a maximum of 1.0uA at 85°C, and is typically about 0.05uA at 25°C. With the worst-case limit of 1uA and Vcc of 5V, I calculate an impedance of 5M Ohms (5V / 1uA = 5M Ohms), and is more typically 100M Ohms.
These numbers are pretty typical of solid state analog switches. The only other part I could identify with the same package, pin-out, and 5V capability is the SN74LVC1G3157DRYR, but its specifications are very similar.
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